1. Field of the Invention
The present invention relates to a sensorless motor driving apparatus which detects a position of rotor without using a Hall sensor and drives a motor so as to rotate in a predetermined direction by controlling the order of energization of phases in accordance with a result of detection.
2. Description of the Related Art
FIG. 8 is a block diagram showing a three-phase sensorless motor driving apparatus commonly used according to the related art. The three-phase sensorless motor driving apparatus shown in FIG. 8 comprises power transistors Tr 801–811, a power supply voltage Vm, a motor 819, comparators 821a–821c, a position detecting circuit 823, and an output transistor control circuit (not shown).
Referring to FIG. 8, U-phase, V-phase and W-phase driving terminals are connected to non-inverting input terminals of the comparators 821a–821c. A center tap of the motor 819 is connected to inverting input terminals of the comparators 821a–821c. Output terminals of the comparators 821a–821c are connected to the position detecting circuit 823. The comparators 821a–821c output binary signals indicating relative magnitudes of a back EMF generated in the U-phase, V-phase and W-phase, and a center tap voltage.
The position detecting circuit 823 determines which of the six transistors Tr 801–811 is to be turned on in accordance with the binary signals output from the comparators 821a–821c, and outputs a logic signal to be supplied to the gate of the transistors Tr 801–811. In accordance with this construction, the binary signals output from the comparators 821a–821c indicate the rotor position. Therefore, the rotor can be smoothly operated by switching between phases for energization in accordance the rotor position.
Accordingly, a commonly used three-phase sensorless motor driving apparatus is constructed such that the rotor position is detected in accordance with the back EMF occurring in phase coils as the rotor rotates, and the motor is driven by switching between phases for energization in accordance with the result of detection.
In this sensorless driving scheme, however, the rotor position cannot be detected and the back EMF does not occur in the phases when the rotor is in a stationary state. In this situation, there is likelihood that the motor may be rotated backward when a driving signal is applied.
A patent document No. 1 discloses an invention related to the present invention. The document discloses that the shaft loss at start-up of a motor is reduced by supplying an excitation current of a frequency higher than the characteristic frequency of the motor to stator coil, prior to the sequential steps of supplying an excitation current at start-up.
[Patent document No. 1]
Japanese Laid-Open Patent Application No. H06-141588
However, it is difficult to prevent backward rotation of a motor even when the invention of the patent document No. 1 is used.
Accordingly, an object of the present invention is to provide a sensorless motor driving apparatus and a driving method in which backward rotation of a motor is prevented.